Currently, worldwide demand for wireless services is growing at an ever quickening pace. This demand is not only for an increased number of users but also for extended wireless access capabilities. These capabilities include for example, Internet access, video conferencing and multimedia applications.
Code Division Multiple Access (CDMA) and Wideband-CDMA (W-CDMA) are spread spectrum based broadband communications technologies that are becoming increasingly popular in mobile wireless communication systems and, in particular, for 3G mobile systems currently under development. In a CDMA system, all users simultaneously occupy the same frequency. In contrast, users in Frequency Division Multiple Access (FDMA) systems communicate over separate frequencies. Users in Time Division Multiple Access (TDMA) systems receive and transmit data during separate predefined time slots. CDMA systems, however, discriminate between users by assigning a different code for each user. In the downlink, CDMA base stations simultaneously transmit signals to multiple subscriber mobile stations over a single frequency band whereby each signal is generated using a different code associated with each user. Several advantages of CDMA systems over other multiple access systems such as FDMA and TDMA systems include greatly increased spectral efficiently and the ability to reduce the affects of signal fading by making use of known path diversity techniques.
In a W-CDMA base station, each information signal associated with a mobile station is multiplied by a unique spreading code sequence. The spreading code sequence is formed by concatenating (i.e. multiplying) a particular scrambling code together with a channelization code unique to each mobile station. Note that in W-CDMA, relatively long scrambling codes (38,400 chips) are concatenated with short channelization codes resulting in the spreading code used to modulate the information traffic. Examples of long codes include complex or non-complex Gold sequences, PN sequences or maximal length sequences. Examples of short codes include Orthogonal Variable Spreading Factor (OVSF), Walsh or Hadamard codes. Multiplication of the information signal by the spreading code sequence creates the spreading of the signal spectrum by increasing the rate of transmission from the bit rate to the chip rate. The spread signals for all users are transmitted simultaneously by the base station.
A W-CDMA signal is de-spread by correlating the received signal with the known spreading sequence. The results are adequate as long as the influence of other users can be neglected. The influence between users is reduced by choosing the spreading codes so that the cross correlation between different codes and multipath delays is low (e.g., Gold, OVSF codes).
At each mobile station, a receiver de-spreads the received signal by multiplying the received signal by the code sequence assigned to the receiver. The de-spreading is accomplished using a correlator which functions to generate the information signal intended for the particular mobile station. The correlator operation is such that signals encoded with other user's codes intended for other mobile stations appear at the output of the correlator as noise.
The structure and operation of spread spectrum systems are well known. See, for example, Robert C. Dixon, “Spread Spectrum Systems,” John Wiley & Sons, 1984. In addition, the structure and operation of CDMA systems are well known. See, for example, Andrew J. Viterbi, “CDMA: Principles of Spread Spectrum Communication,” Addison-Wesley Publishing, 1995.
The use of W-CDMA has been proposed for applications requiring higher bandwidth such as multimedia applications. W-CDMA achieves higher data rates by using higher chip rates for the spreading waveform resulting in a higher ratio of chip rate to information rate. Considering that the main interference in a W-CDMA system is from other users, the increased spreading factor or processing gain allows for improved handling of higher interference levels, which translates to a higher number of users that can be supported in the same cell. In addition, the reduced chip period results in more multipath components being separated by at least one chip period making more paths available to be resolved by the receiver.
In CDMA systems and especially W-CDMA systems, rake receivers are typically employed to combat multipath interference. A rake receiver exploits the path diversity present in the input RF signal. The transmitted signal typically travels to the receiver over a channel that includes many independent paths or multipaths. Each multipath represents a separate route the signal took in traveling from the transmitter to the receiver. A plurality of multipath signals arrive at the receiver with each multipath having a different delay, phase and signal strength due to the fading present in the channel.
Narrowband multiple access techniques such as FDMA and TDMA cannot discriminate between the individual multipath signals that arrive at the receiver because their symbol transmission times are too long, meaning their duration is relatively long making it impossible to discriminate between individual multipath components. Instead, these systems resort to equalization to combat the negative effects of multipath interference.
The function of the rake receiver in a W-CDMA system is to discriminate between individual multipath signal components, demodulate them and combine them to produce a stronger output signal. Separate fingers are used to receive and demodulate the different multipath signals. The energy received from each finger is combined resulting in a stronger received signal. A searcher functions to search for the strongest multipath signals and assigns the fingers to those multipath components.
In a rake receiver, a single finger may be required to perform several correlations. Typically, these include correlations of a given code with several adjacent signal phases (e.g., early, on-time and late). If a common pilot channel is included in the transmission, it is used to perform channel estimation in addition to receiving the data signal. The common pilot channel must be processed for each data signal using a different phase and delay similar to the processing for the data signal. Thus, there is a need to allocate the same number of fingers for pilot signal processing as for data channels. In the event many data signals are transmitted, the channel estimation processing requires numerous correlations as well. Many correlations are also required in the case when several data signals have the same phase and delay but use different codes. The number of correlators required increases linearly as the number of unique codes, phases and delays the input signal is to be correlated against increases.
A block diagram of a prior art W-CDMA rake receiver comprising a plurality of fingers is shown in FIG. 1. The receiver, generally referenced 10, comprises a plurality of fingers 12. Each finger is configured to process a different multipath component and comprises an independent correlator. The correlator comprises a multiplier 14 adapted to multiply the input Rx signal by a spreading code 20. The multiplication result is then integrated over the symbol duration, i.e. the spreading factor, via adder 16 and register 18 functioning as an accumulator. It is important to note that the multiplier and adder is duplicated in each finger. Thus, N fingers requires N multipliers and N adders to construct.
Prior art implementations of rake receiver fingers require allocating individual correlators for each signal having different phase, delay, code, pilot channel, etc. The need for such a large number of correlators requires the allocation of sufficient hardware processing resources, e.g., gate count, chip count, etc., with the resultant increase in power consumption, size, cost and complexity and a consequent decrease in reliability.
There is thus a need for a reduced complexity correlation mechanism capable of performing a plurality of correlations that does not require considerable additional hardware as the number of correlations required increases. In addition, the correlator should be capable of reusing one or more correlator components thus greatly reducing the number of required gates to construct the correlator.